In a gas-insulated apparatus such as a gas-insulated switch gear or a gas-insulated main bus, at least one conductor is arranged inside a sealed vessel, which is filled with insulating gas having good insulation properties and put into a practical use. To measure the current flowing through the conductor inside the sealed vessel, a wound-type current transformer or an optical current transformer is usually used.
In the optical current transformer for a gas insulating apparatus described in Japanese Patent Application Laid-Open No. Hei 10-142265 (Patent literature 1) for example, an optical fiber, which is used as a current sensor, is annularly wound on each of three conductors arranged inside a sealed tank, and both ends of each optical fiber are led out of the sealed tank. With this configuration linear-polarized light emitted from a light-emission unit enters the optical fiber and outgoes therefrom with its polarization rotated because of Faraday effect of optical fiber. The angle of such polarization is detected with a measuring unit that functions as an optical current transformer for a gas insulating apparatus to determine the current flowing through.
In this style of optical transformer for a gas-insulate apparatus, airtightness at the leading-out portion should be assured in leading out the optical fiber from the sealed tank to prevent leakage of insulating gas. For this purpose, Japanese Patent Application Laid-Open No. Hei 8-211232 (Patent literature 2) for example has proposed providing a hermetic junction at the optical fiber leading-out portion on the sealed tank.
The hermetic junction described in Patent literature 2 being named an air tight adapter is used under the situation in which a cylindrical hollow flange is formed on the outer face of the sealed vessel in a protruding manner and a lid plate having a through-hole in the center thereof is detachably fixed on the cylindrical hollow flange. The air tight adapter has: a disc-like flange part placed so that the through-hole on the lid plate will be closed thereby, in which the disklike flange part is bored with an insertion hole to permit inserting an optical fiber; and an optical fiber, partly having metallic film thereon, inserted into the insertion hole, in which the space between the insertion hole and the metallic film is sealed and fixed with a material like solder.
In the case that an optical current transformer for gas insulated apparatus is composed using the air tight adapter defined in Patent literature 2, if the optical fiber requires replacement due to aging deterioration or similar reason, the replacement operation needs to remove the lid plate from the cylindrical hollow flange on the sealed vessel. In this operation, there is a problem in that the insulation gas in the sealed vessel must be treated first and thereafter the replacement of the optical fiber becomes practicable.
Further, the use of the air tight adapter of Patent literature 2 as a hermetic junction demands to change the number of the air tight adapters to a change-matched number when the protection system is required to be changed from a single system to a dual system or when the number of optical fibers is to be varied because of a requirement for increasing the optical current transformer, because such air tight adapter can fix is only one optical fiber. Moreover, dimensions of the cylindrical hollow flange, which is formed on the outer face of the sealed vessel in a protruding manner, or of the lid plate is to be varied depending on the size of the disc-like flange part having the insertion hole. Therefore, there has involved a problem in the use of such device in that, in an extreme case, the main circuit of the gas insulated apparatus has to be disassembled to permit replacing the sealed vessel.
An object of the present invention is to provide an optical current transformer for gas-insulated apparatus that permits, with a simple structure, disposing an optical fiber without emitting insulating gas in a sealed vessel and further permits an eased operation for increasing/decreasing the number of optical fibers.